Field of the Invention
The present invention relates to a sulfide-based solid electrolyte, and a secondary battery including the solid electrolyte.
Description of the Related Art
Based on sodium, which is a low environmentally-hazardous material, sodium secondary batteries are expected as low-cost next-generation batteries. In sodium-sulfur batteries (NaS batteries) used for day and night load leveling, etc. as a large-scale power storage, β-alumina has been used as a solid electrolyte up until now. In order for the solid electrolyte to maintain sodium ion conductivity, the operation temperature of the battery has been limited to a high temperature equal to or higher than 300° C.
In this context, use of Na3PS4, which is a material having a high conductivity of 10−4 Scm−1 at room temperature, as a solid electrolyte has made it possible for sodium-sulfur batteries, which have been able to operate only at a high temperature, to operate at room temperature (see “A. Hayashi, K. Noi, A. Sakuda, and M. Tatsumisago, Superionic Glass-Ceramic Electrolytes for Room-Temperature Rechargeable Sodium Batteries, Nature Communications, 3 (2012) 856: 1-5”). However, the ionic conductivity of a solid electrolyte of lithium secondary batteries, which are expected as power storage batteries likewise, is about 10−3 Scm−1, which is higher by one digit (see International Publication No. WO 2011/118801). Such a high ionic conductivity that lowers the resistance in the battery is very advantageous for apparatuses requiring a high current.
Hence, it is requested to further improve the Na ion conductivity of a solid electrolyte of sodium secondary batteries, likewise.